1. Field of the Invention
This invention relates to an anti-reflection member and manufacturing method thereof, and relates to a cathode ray tube having a front panel applied with such anti-reflection member.
2. Description of Related Art
For example for conventional cathode ray tubes (CRT), a picture displayed on the cathode ray tube is not viewed clearly because external light is reflected on the panel surfacel; that is a problem of conventional cathode ray tubes. On the other hand, the potential of the panel surface changes because of high voltage applied on the fluorescent screen formed on the inside surface of the panel of the cathode ray tube. As a result, dust adheres on the panel surface and the panel causes an electric discharge to an approching human body. To prevent such phenomena, it is required to provide an antistatic function on the panel surface. In addition, it is also important to provide an electromagnetic radiation shielding function so as not to release electromagnetic waves from the cathode ray tube. As a method for solving all the technical problems as described herein above, a technique in which an anti-reflection member having conductivity is applied on the panel surface has been known.
Such an anti-reflection member comprises a laminate composed of a substrate, a hard coat layer, and a multilayered reflection preventing optical thin film. The substrate consists of, for example, polyethyl-eneterephthalate (PET) or polycarbonate (PC). A hard coat layer consists of, for example, polymethyl-methacrylate (PMMA), and is formed on the surface of the substrate because the surface is susceptible to touching. In the case that it is not necessary to provide an antistatic function and an electromagnetic radiation shielding function to an anti-reflection member, at least two layers of dielectric thin film consisting of SiO, SiO.sub.2, TiO.sub.2, ZrO.sub.2, Ta.sub.2 O.sub.5, and Y.sub.2 O.sub.5, for example, and a high refractive index film/low refractive index film/high refractive index/low refractive index film . . . ) are formed on the hard coat layer. Materials having high refractive index (having a refractive index of 1.8 to 2.7 at around the wave length of 550 nm) such as TiO.sub.2, ZrO.sub.2, Ta.sub.2 O.sub.5, and Y.sub.2 O.sub.5 are excellent in adhesion with hard coat layers consisting of polymethyl-methacrylate.
On the other hand, in the case that it is necessary to provide an antistatic function and an electromagnetic radiation shielding function to an anti-reflection member, it is preferable to structure a reflection preventing optical thin film by laminating a transparent conductive oxide film and a thin film (referred as to low refractive index film hereinafter) consisting of a material having lower refractive index than that of the material used for the transparent conductive oxide film. By structuring the reflection preventing optical thin film as described herein, not only is antistatic function and electromagnetic radiation shielding function due to transparent conductive oxide film provided to the anti-reflection member but also an anti-reflection function due to the high refractive index film/low refractive index film structure is provided simultaneously. Thereby the structure of a reflection preventing optical thin film is simplified. An example of the transparent conductive oxide film includes ITO (I.sub.2 O.sub.3 doped with Sn) film (refractive index is 1.9 to 2.0). On the other hand, examples of the low refractive index film include SiO.sub.2 film (refractive index=1.46 at 550 nm) and MgF.sub.2 film (refractive index =1.38 at 550 nm).
Usually, by repeating the film forming process, namely, ITO film/low refractive index film/ITO film/low refractive index film . . . on a hard coat layer formed on a substrate, an anti-reflection member is provided with an anexcellent anti-reflection function in the wide wave length range from 450 to 650 nm. A schematic partial cross-sectional view of the anti-reflection member having such structure is shown in FIG. 4A. In FIG. 4A, a reflection preventing optical thin film having a four layer structure is shown; the first layer and third layer consist of ITO film, and the second and fourth layer consist of low refractive index film. Usually, ITO film is formed by spattering using a target of oxide ITO. An ITO film is formed under the spattering condition that the composition of such ITO target is, for example, In.sub.2 O.sub.3 /SnO.sub.2 =90 parts by weight/10 parts by weight, target current density is 2 W/cm.sup.2, and others are Ar/O.sub.2 =50 volume %/50 volume %, and 0.2 Pa.
It is reported that the crystallization temperature of ITO ranges from 150.degree. to 200.degree. C. Therefore, to improve the adhesion (adhesion strength) to a hard coat layer, it is desirable to heat the substrate to a temperature of 120.degree. C. or higher. However, such heating of a substrate can cause thermal damage such as deformation of the substrate consisting of plastic material. Heating of a hard coat layer consisting of polymethyl-methacrylate to a temperature of 100.degree. C. or higher can cause the reduction of the hardness.
Recently, ITO film formed on a color filter was used as a transparent electrode for a flat panel display of liquid crystal display devices. Such an ITO film is formed by DC magnetron spattering using ITO target. ITO film formed by such method is excellent in resistivity, wet etching performance, and reproducibility of characteristics. However, an ITO film formed on a hard coat layer by DC magnetron spattering using an ITO target is insufficient in adhesion strength to the hard coat layer.
The inventors of the present invention proposed a new anti-reflection member comprising a laminate of a substrate, a hard coat layer, and a multilayered reflection preventing optical thin film in Japanese Patent Application Hei 7-170925 (application date: Jul. 6, 1995). A schematic partial cross-sectional view of the anti-reflection member having such structure is shown in FIG. 4B. The substrate of the anti-reflection member consists of, for example, polyethylene-terephthalate (PET) and polycarbonate (PC). A hard coat layer consists of, for example, polymethyl-methacrylate (PMMA) an is formed thereon. To provide an antistatic function and an electromagnetic radiation shielding function to the anti-reflection member, the anti-reflection member has the first layer of the reflection preventing optical thin film comprising a conductive light absorbing film, and the second layer of the reflection preventing optical thin film consisting of a material having lower refractive index than that of the material used to structure the first layer.
The conductive light absorbing film consists of a material selected from the group composed of Ag, Au, TiN.sub.X (X=0.3 to 1), TiO.sub.X N.sub.Y (wherein X=0.3 to 1, Y&lt;1, and Y.ltoreq.X), TaN.sub.X (wherein X=0.2 to 1), Pt, Al, Cu, Ta, Ni-Cr, Cu-Al, Cu-Zn-Al, Cu-Ni-Al and Cu-Sn-Al, and the thickness ranges from 4 to 40 nm. On the other hand, the second layer of the reflection preventing optical thin film consists of SiO.sub.2 or MgF.sub.2. The above-mentioned material to be used for the conductive light absorbing film has a characteristic that light absorption coefficient changes dependently on wave length. Reflection is prevented for a wide range of wave length (430 to 650 nm). This is possible even if the reflection preventing optical thin film has a two layer structure. The light transmittance of these materials ranges from 70 to 90%, however, because the two layer structure can be used for the reflection preventing optical thin film, the low light transmittance of the conductive light absorbing film does not cause any problem.
However, it was found that the adhesion (adhesion strength) of such conductive light absorbing film to a hard coat consisting of polymethyl-methacrylate was insufficient.
Therefore, it is the object of the present invention to provide an anti-reflection member provided with an antistatic function and an electromagnetic radiation shielding function which has a reflection preventing optical thin film excellent in adhesion to a hard coat layer, and a manufacturing method thereof. In addition, a cathode ray tube having a front panel applied with such anti-reflection member is provided.